1. Field of the Invention
The invention broadly pertains to a method for cooling dry elastomeric polymer particles. More specifically, the invention relates to a method for enhanced cooling of dry rubber crumb by transferring the rubber particles from a first stage pneumatic conveyor with a hot wet air stream to a second stage pneumatic conveyor with a cool dry air stream by means of an interstage cyclone separator which transfers the particles form one fluid stream to the other.
A typical elastomer drying process utilizes a devolatizing extruder discharging to a pneumatic conveying system. The rubber particles are cooled in the pneumatic conveyor, and are subsequently conveyed to and discharged into an enclosed vibrating conveyor. The rubber particles are cooled further in the vibrating conveyor, and thence are transported via a series of open vibrating conveyors to a baling or packaging area.
The level to which the rubber crumb is able to be cooled prior to exiting the pneumatic conveyor is limited by the saturation temperature of the air and steam mixture in the pneumatic conveyor. If the air temperature is lowered below the saturation temperature, the elastomeric product is undesirably wetted by the free water which is formed.
The foregoing limitation, together with the peculiar properties which characterize the rubber crumb, give rise to several problematic deficiencies which result in system performance and quality concerns. Because the rubber particles entering the enclosed vibrating conveyor from the pneumatic conveyor are still hot, the particles stick to the conveyor surface and agglomerate, thereby causing fouling of the equipment downstream of the pneumatic conveyor. Additionally, the hot rubber which causes the fouling tends to degrade and break away, resulting in contamination of the product. Moreover, the resulting rubber agglomerations cause weight-control irregularities in the packaging area.
Hence, the need exists for a process for cooling rubber particles in an elastomer drying system to a greater extent than the cooling presently achievable in the pneumatic conveyor, prior to presentation of the particles to the enclosed vibrating conveyor. The present invention realizes the preceding object by providing a method, wherein the rubber is transferred from the water-saturated air stream of the first stage pneumatic conveyor, to the cool, dry air stream of a second stage pneumatic conveyor by means of a unique interstage cyclone separator. Once the rubber particles are introduced into the cool dry air in the second stage pneumatic conveyor, the particles are subject to additional cooling over that which is able to be obtained in the first stage pneumatic conveyor. Indeed, the rubber particles may be cooled in the second stage pneumatic conveyor to such an extent that the enclosed vibrating conveyor, and its attendant disadvantages, may be entirely eliminated.
The process of the subject invention, therefore, allows rubber crumb in an elastomer drying system to be cooled and transported to the baling or packaging area in a pneumatic conveying system, without the need for the extraneous cooling and conveying equipment which is presently utilized. Consequently, the product contamination currently associated with the vibrating cooling and conveying equipment is eradicated. Furthermore, the two-stage pneumatic conveying process allows potentially toxic compounds from the devolatizing extruder to be more easily and successfully contained in the discharge air, and offers future potential for adaptation to grades of rubber product which are incompatible with conventional cooling systems.
2. Description of the Prior Art
It is known in the prior art to utilize pneumatic conveyors for transporting or transferring particulate material. For example, U.S. Pat. No. 3,384,420, issued on May 21, 1968 to Fiscus discloses a transfer system wherein particulate material is fed to a hopper, and thence to a tubular conduit system containing an air stream which conveys the material to a separator, for separation of the particulate material from the air stream.
Similarly, U.S. Pat. No. 4,599,016, issued on July 8, 1986 to Mendemblik, teaches a pneumatic conveying apparatus for granular matter comprising a first cyclonic separator for receiving the matter conveyed by an air stream, and a secondary cyclonic air cleaner for removing dust and abrasive particles from the air stream. The granular matter is conveyed from the first cyclonic separator by means of an air stream.
The prior art further discloses diverse methods and means for drying a variety of materials by causing the material to contact a drying medium. For instance, U.S. Pat. No. 2,436,355, issued on June 5, 1946 to Cadot et al, is directed to a process for spray drying dimethylolurea wherein slurry is fed into a chamber to which cold air is admitted in order to effect crystallization or separation of the product.
U.S. Pat. No. 4,365,057, issued on Dec. 21, 1982 to Saito et al. discloses a method for drying polyolefins which are caused to descend in a drying apparatus while contacting counterflowing nitrogen gas.
U.S. Pat. No. 3,629,951, issued on Dec. 28, 1971 to Davis et al, is directed to a spray-drying method for an aerated slurry wherein hot air is provided to the spray-drying chamber, and wherein the dried granules are removed from the chamber onto a conveyor.
Various methods and means for cooling particulate material are also established in the prior art. Exemplary of such teachings is U.S. Pat. No. 4,441,261, issued on Apr. 10, 1984 to Beckmann. The latter patent teaches a method and apparatus for cooling hot bulk material in a cooling bunker by passing cooling gas streams therethrough.
U.S. Pat. No. 4,231,991, which issued to Muller on Nov. 4, 1980, discloses a process and apparatus for cooling lime introduced into a cooling tower. Cool air is drawn into the cooling tower and directed to flow up through the falling lime.
U.S. Pat. No. 4,076,493, issued on Feb. 28, 1978 to Gardner shows a cooling apparatus for particulate material including a shell for receiving the material and air distributing means for releasing cooling air into the material for movement counter to the direction of material flow.
Finally, U.S. Pat. No. 3,265,775, issued to Friedrich on Aug. 9, 1966, is directed to means for cooling and/or drying a granular material by treating the material with a gas flowing in counterflow to the material within a vessel.
It is apparent from the foregoing that the prior art fails to teach or suggest a process for enhanced cooling of an elastomeric material characterized by a two-stage pneumatic conveying system utilized in conjunction with an interstage cyclone separator for transferring the material from a hot wet air stream to a cool dry air stream.